Atomic force microscopes (AFM's) were developed, among other reasons, to meet a demand in the semiconductor industry for accurately measuring critical dimensions (CDs) during integrated circuit (IC) fabrication. Critical dimensions constitute the width of a line or space on a substrate identified as crucial for proper operation of the device being fabricated. Critical dimensions are measured on some or all of a batch of substrates following any photolithography or etching process in which the dimensions are critical. A critical dimension may be the width of a patterned line, the distance between two lines or devices, or the size of a contact.
Due to their inherently high spatial resolution, AFM's are well suited for measuring critical dimensions nondestructively during process stages of IC fabrication. Generally, AFM's are equipped to sense atoms on or in sample surfaces, thereby providing atomic level surface imaging for measuring critical dimensions at a significantly higher resolution than comparable measurements taken from cross-sectional scanning electron micrographs acquired before and after an IC fabrication stage. AFM images are reconstructed from digital data acquired during grid scanning of a probe tip across the sample surface. Probe tips for measuring critical dimensions must be capable of resolving nanometer size structures.
Conventional processes for fabricating probe tips capable of measuring critical dimensions suffer from several disadvantages that limit not only process yield, but the quality of the probe tips as well. In particular, CD probe tips exhibit a limited cantilever-to-tip height, which negatively affects AFM performance due, for example, to low quality factors and high squeeze film dampening. In addition, conventional fabrication processes for CD probe tips may rely on a wet etching process, which restricts control over tip shape and makes batch processing difficult. Another conventional fabrication process for CD probe tips relies on dry etching with a plasma formed from a source gas composed primarily of a fluorine-based gas, such as sulfur hexafluoride (SF6). However, such conventional dry etching processes produce extraneous projections at locations other than the intended locations for the probe tips. The presence of these extraneous projections lowers process yields, which is a significant hindrance to batch fabrication of probe tips. As a result, conventional probe tips are manufactured individually with multiple tedious inspections made using scanning electron microscopy at various stages of the manufacturing process to verify tip quality and to adjust, if necessary, the dry etching process recipe.
What is needed, therefore, are probes and probe tips suitable for use in an atomic force microscope to measure critical dimensions and methods of accurately manufacturing such probes and probe tips that overcome these and other deficiencies of conventional probes and probe tips and conventional manufacturing methods.